Monthly Archives: February 2010

We’ll end the work week with a deskcrop of Quaternary basalt from the McKenzie Pass region of the central Oregon Cascades. This sample is from a relatively young (~1.5 ka) lava flow, just east of the Dee Wright Observatory on Oregon Highway 242. The sample has both vesicular and porphyritic textures, and thought the former didn’t photograph well, one can see small phenocrysts of both olivine and plagioclase, especially if you zoom in to a larger version of the photograph.

For today’s deskcrop we’ll revisit the Mississippian Muleshoe Mound in the Sacramento Mountains, just southeast of Alamagordo, New Mexico. Like my previous deskcrop from this locality this sample is chock full of crinoid stems, but even more prominently featured is the large brachiopod located in the center of the sample.

Part of my Ph.D. field work brought me to the Eagle Rest Peak intrusive suite located in the San Emigdio Mountains of the Sierran Tail, southwest of Bakersfield, California. Much of this Jurassic intrusion here is gabbroic, with minor ultramafic units, and a fair amount of tonalitic rocks, as well. The deskcrop below is in no way representative of the complex as a whole – to the best of my recollection it’s the only pegmatitic rock I collected there, but it quickly became a favorite teaching piece for its large, well formed hornblende crystals. It’s hard to illustrate the excellent 56o-124o cleavages that these crystals show in a photograph, but you can see a hint of it in the pseudo-diamond cross section of the crystal near the center of the rock.

Following yesterday’s theme of not-quite-economic resources, today’s deskcrop is a piece of nearly pure chromite ore (chromatite) from the Stillwater Intrusive Complex in the Beartooth Mountains of southern Montana. Last mined economically during the Korean War, the chromite deposits of the Stillwater Intrusion are not nearly as economical to mine today as those of the massive Bushveld Intrusion of South Africa. Nevertheless, they are part of a much more accessible locality for North American geologists who wish to study the field relationships and igneous processes of layered mafic intrusions (LMIs).

We’ll start the work week off with a deskcrop of oil shale from the Eocene Green River Formation, a little bit northeast of Douglas Pass, Colorado. When I originally collected this piece a freshly fractured surface was noticeably aromatic. We even burned a couple pieces in our campfire during field camp, though the result was smellier than it was flammable. To the best of my understanding this is not the locality of the truly high grade “mahogany ledge” in the Green River Formation, though this locality is worth the detour for its preservation of leaf and insect fossils. If anyone knows of an exposure where one can collect a piece of the “mahogany ledge” I’d sure like to hear about it so I can add that to my deskcrop/teaching collection.

In the eastern portion of Badlands National Park, while watching out for rattlesnakes my eye was caught instead by this lovely little dry stream bed with some of the most beautiful asymmetric current ripplemarks I’ve ever seen (and even some mudcracks, to boot). I would’ve loved to have been able to lithify this sample and bring it back to the lab for teaching purposes, but alas, photos were all I was able to bring back…

Dry Streambed

Asymmetric Current Ripplemarks

Asymmetric Ripplemarks (and Mudcracks) – Flow Direction Was Right to Left

As the Alameda Parkway traverses the Dinosaur Ridge hogback on the west side of Denver, CO roadcuts expose a wonderful array of sedimentary rocks, including the famous Jurassic Morrison Formation, not far from its type locality. This weekend I want to highlight some of the beautifully preserved ripplemarks near where the Parkway crosses the crest of the ridge. We’ll look at some younger ripplemarks tomorrow…

Linear Ripplemarks

Interference Ripplemarks

Ripplemark Profile

As a geologist I’m usually focusing my camera at the rocks and occasionally cursing the flora that gets in the way. However, I’m not averse to taking photos of pretty flowers – especially when they serve to illustrate a geologic feature. This one was just a stone’s throw from those lovely ripplemarks…

It feels like forever since we’ve visited the Adirondacks (actually it’s only been about three weeks). It’s certainly been too long since I posted a good metamorphic deskcrop. Well, let’s fix that…

Today’s deskcrop comes from the old mine dump at Willsboro, NY. This deposit is one of a number in the eastern Adirondacks that are/were mined for their wollastonite-diopside-garnet skarn rocks. I know that eclogites are more frequently described as “Christmas tree” rocks, but this beauty would certainly be an admirable alternative. I’ve included three views of the sample so you can see the uneven distribution of the three principal minerals.

The Red Hills of southern Kansas are made of Permian aged redbeds and are so riddled with gypsum veins, they’re also known as the Gypsum Hills. Near Medicine Lodge, Kansas this rock is even mined for gypsum to make wallboard. Today’s deskcrop is one of the gypsum vein fragments that made it back to my office. You can still see the very red soil, which I haven’t entirely washed off.

Gypsum Vein

Gypsum Vein – Side View

Even though its still a weekday, I’ll throw in an outcrop GigaPan for good measure. Actually I thought I had already published this one on my blog, but apparently I had not.

Today’s deskcrop is for all the Michiganders out there – your state rock: Petoskey Stone. This distinctive looking rock is actually a Devonian colonial coral, Hexagonaria percarinata. Often sold in rock shops as a polished pebble, this sample is a rough one from the LaFarge limestone quarry near Alpena, MI.